1. Field of the Invention
The present invention relates to a multi-level selecting circuit advantageously applicable to, for example, a driver for driving a liquid crystal display (LCD).
2. Description of the Prior Art
An LCD driver, for example, may be constructed to generate three or more different voltages, select one of such voltages at a time, and feed the selected voltage to the LCD, as has been customary in the art. A selecting circuit is built in this type of LCD driver and is often implemented as a multi-level selecting circuit having a plurality of MOS transistors. Specifically, a multi-level selecting circuit has a plurality of MOS transistors each connecting to respective one of input lines to which a plurality of different levels are applied. As any one of the transistors is turned on under external control, one of the different input levels associated with the conductive transistor is fed to the LCD.
The MOS transistors of the above-state multi-level selecting circuit are integrated on a single substrate by CMOS process technologies. Specifically, a plurality of p-MOS transistors and a plurality of n-MOS transistors are arranged on a single substrate. To form the p-MOS transistors, p-channels may be directly provided in the surface layer of an n-type substrate. Regarding the n-MOS transistors, a p-well is formed in the n-type substrate by ion injection, and then n-channels are provided in the surface layer of the p-well.
The problem with the integrated circuit having the well in a substrate is the occurrence of latch-up. Specifically, a parasitic transistor and a parasitic resistor are formed between the substrate and the well and between the well and the channels, constituting a parasitic thyristor which is the cause of latch-up. Latch-up refers to a phenomenon that when an overvoltage is applied to the input side of the circuit, the parasitic transistor is rendered conductive to cause a great current to flow through the circuit. This problem is especially serious when it comes to an LCD driver, as follows.
An LCD driver has a great number of multi-level selecting circuits connected to a level output circuit thereof. Hence, an overcurrent is apt to flow through the MOS transistors of the last-stage multi-level selecting circuit, causing the selecting circuit to latch up. When any one of the selecting circuits latches up, the MOS transistors thereof malfunction to disturb the operation of the LCD. No countermeasures against such latch-up have been proposed in the past in relation to a multi-level selecting circuits which drives an LCD or similar load.